This disclosure relates to the area of Uplink Power Control, UL PC, and Downlink Power Control, DL PC. UL PC and DL PC are relevant for several different Radio Access Technologies, RATs. The solutions described herein will be exemplified with regards to Long Term Evolution, LTE, wireless networks but the solutions are applicable to other types of wireless communication networks or RATs as well.
In wireless communication networks, the UL is typically a challenging link, where the available scarce energy of a wireless device, e.g. a User Equipment, UE, must be used to compensate for the losses of the channel (distance dependent pathloss, shadow fading, fast fading, etc.). Moreover, the interference produced by any UL transmissions in a multi-cell environment is also another limiting factor for the UL performance. One way to use efficiently the available energy at the UE is to control the UL transmit power. UL power control can be used on both data and control channels. Also the DL may be a challenging link. A wireless access point, e.g. an eNodeB or Radio Base Station, RBS, may not be limited with regards to transmission power, but the interference situation may cause the wireless access point to moderate its DL transmission power accordingly.
UL power control in LTE is a topic discussed earlier and part of already the first 3rd Generation Partnership Project, 3GPP, LTE standard release, e.g. Rel. 8. According to the standardised method, UL PC is typically based on compensating the pathloss to the connected cell. For example, a UE which is close to the BS will use less transmit power with respect to a UE which is close to the cell edge. This power control principle does not take into account any impact of the selected transmit power on surrounding (or interfered) cells.
The UL and DL power control may become even more intricate in heterogeneous networks, where different wireless access points of different downlink transmission powers are employed. The size of the respective coverage areas, also referred to as cells, for wireless access points of different downlink transmission powers may vary substantially and also the total number of UEs, the density (i.e. the number of UEs per area unit of a cell) may vary substantially. Uplink power control in such heterogeneous networks plays an important role: it balances the need for sufficient transmit power to maintain the required Quality-of-Service, QoS, against the need to control inter-cell interference and maximise the UE battery life.
In achieving this goal, an efficient power control algorithm has to adapt to the characteristics of the radio propagation channel by taking into account path-loss or geometry conditions as well as overcoming interference from other users in neighbouring cells.
In LTE, uplink power control is a combination of two terms: a basic open loop operating point for compensating slow changes in pathloss, and a closed loop mechanism consisting of explicit control commands transmitted in the downlink for user specific power adjustments.
According to 3GPP, the transmit power target per resource block (PR) for PUSCH transmission can be evaluated as PSDTX=P0+αPL+δCL, where P0 is the received power target (user or cell specific), α is the path-loss compensation factor (cell specific), PL is the downlink path-loss measured by the UE and δCL is the closed loop component.
Since the parameters P0 and a determine the open loop operating point, they can be used by the operator to control the uplink power. Therefore, different choices of the parameters P0 and a can lead to different UL power control configurations.
In single-cell configurations, the parameters that define the open loop operating point are set by utilizing information related only to the serving cell. The most common single-cell configurations are the pathloss-based power control method, which is also the 3GPP baseline, and the load based power control method.
The fractional path-loss compensation factor α is a cell-specific parameter that can be seen as a tool to control the trade-off between cell-edge data rate and total uplink capacity.
Uplink power control with α=1 corresponds to full pathloss compensation. Full path-loss compensation maximizes fairness for cell-edge users by adjusting the UL power so that the received power remains constant.
On the other hand, by setting α<1 the UE compensates only a fraction of pathloss when setting the transmit power. In this way, fractional pathloss compensation (FPC) can improve the total system capacity in the uplink by assigning relatively lower transmit power to the terminals close to the cell border (higher path-loss), so that cell-edge UEs cause less inter-cell interference. Typically, path-loss compensation factors around 0.8 have been shown to give a close-to-optimal uplink system capacity without degrading significantly the cell-edge data rate.
An assumption in Fractional Power Control, FPC, is that UEs with low pathloss can increase their transmit PSD without causing too much interference. However, this assumption is only valid in homogeneous scenarios. For example, in scenarios with mixed indoor and outdoor users, increasing the power of an outdoor UE with relatively low pathloss can cause strong interference to a neighbouring indoor UE that has relatively high pathloss. In this case, it is highly possible that the indoor UE becomes power limited when it is close to the cell border and has no power to boost Signal to Interference and Noise Ratio, SINR.